Method for coherent watermark insertion and detection in color halftone images

ABSTRACT

A method for generating an authenticable color image, the color image including a plurality of color separations, wherein an authentication image inserted in the multicolor image is not readily visually perceptible, includes halftoning image data corresponding to a first color separation using a single halftone screen, wherein the halftone screen includes means for generating an authentication image in a color image; and halftoning image data corresponding to a second color separation using the halftone screen and dot placement information for the image data corresponding to the first color separation to form a multicolor image; wherein halftoning of image data corresponding to the second color separation includes placing dots for the second color separation in thresholds of the halftone screen relative to those thresholds occupied by the first color separation in the halftone screen in accordance with a predetermined relationship.

This application relates generally to methods of embedding digitalwatermarks in printed halftone images, and more particularly, to amethod for embedding color digital watermarks in printed halftoneimages.

BACKGROUND & SUMMARY

Watermarks have long been used in the printing industry to identify thesource or origin of a document. Generally, a watermark appears as afaint pattern in an image, which is visible only when the originaldocument is viewed in a particular manner. Unless a counterfeiter hadaccess to the watermarked paper, it would be difficult for him toreproduce the document without showing its inauthenticity. That is tosay, without the paper on which the original image was originallyprinted, the copy should be readily detectable. However, as people moveaway from the use of watermarked papers for cost and other practicalreasons, it is still necessary to identify the source or origin of adocument image.

The introduction of the plain paper copier has resulted in aproliferation of paper copies of paper originals. A similar result ishappening to electronic images, given the easy availability of digitalscanners and quick and widespread access to images throughout theInternet. It is now very difficult for the creator of an image togenerate an electronic original, for which he can be assured thatillegal copies will not be spread to third parties. The use of a digitalwatermark is a technology that aims to prevent that spread, byincorporating an identifying mark within an image that allows one toidentify the source of the image in an electronic copy. It is importantthat the identifying mark not be disturbing or distracting to theoriginal content of the image, while at the same time, allowing easyidentification of the source. The watermarks could be added either bythe scanner, as or after the image is acquired, or by the halftoningsoftware.

Watermark identification may be accomplished by embedding a digitalwatermark in a digital or printed page that will identify the owner ofrights to the image. Watermarking can take two basic forms, visible orperceptible, and invisible or imperceptible. Visible watermarks aremarks such as copyright logos or symbols or logos that are imprintedinto the digital or printed image to be distributed. The presence of thewatermark is made clearly visible in the image or rendered document in away that makes it difficult to remove without damaging the image ordocument. The presence of the visible watermark does not harm theusefulness of the image, but it deters the use of the image withoutpermission. However, visible watermarks may interfere with the use ofthe image or with the image aesthetics. The visible watermark is also apotential target for fraud, in that it is possible for a fraudulentcopier of the image to identify the location of the watermark andattempt to reproduce the image without the watermark or try to transferthe watermark to another image.

Invisible watermarks are marks such as copyright symbols, logos, serialnumbers, other identifiers, etc. that are embedded into digital orprinted images in a way which is not easily discernible or perceptibleby the unaided eye. At a later time, the information embedded in thesewatermarks can be derived or “retrieved” from the images to aididentification of the source of the image, including the owner and theindividual to whom the image is sold. Such watermarks are useful forestablishing ownership when ownership of an image is in dispute. Theywould be less likely to be useful as a deterrent to the theft of theimage. While either or both visible or invisible watermarks aredesirable in an image, they represent different techniques for eitherpreventing copying or detecting copying. It is anticipated that documentproducers may wish to use both kinds of protection.

Embedded watermarks in printed halftone images, which can subsequentlybe detected using a visual aid or using a watermark detection algorithmon a scan of the image are of interest in wide range of applications.U.S. Pat. No. 5,790,703 to Shen-ge Wang for “Digital Watermarking UsingConjugate Halftone Screens,” describes a method for generatingwatermarks in black and white halftone printing using stochasticscreens. In U.S. Pat. No. 5,790,703, monochrome digital watermarks areembedded as correlations in the halftone screen. U.S. Pat. No. 6,731,409to Shen-ge Wang for “System and Method for Generating Color DigitalWatermarks Using Conjugate Halftone Screens” extends the method of U.S.Pat. No. 5,790,703 to color printing. The method disclosed in U.S. Pat.No. 6,731,409 generates color watermarks by producing a halftone patternin one or more color separations of the color document using a separatehalftone screen for each separation. While the color contrast watermarksproduced by U.S. Pat. No. 6,731,409 work well on digital bit-maps, thecolor watermarks are harder to detect in printed hardcopy. The contrastand signal-to-noise ratio of the color watermark produced can be quiteweak because of screen interactions making it impractical for highresolution color printing applications. It is also harder to determinethe shift required for performing a watermark estimation when detectingthe watermark pattern in scanned images due to the use of differentscreens for the different color separations. What is needed is a methodof generating color watermarks having good contrast and goodsignal-to-noise ratios for detection on scans of printed images.

Disclosed in embodiments herein is a method for generating color digitalwatermarks, which provides good contrast while maintaining goodsignal-to-noise ratios. Instead of using a separate screen for eachseparation, the method uses a single halftone screen for all colorseparations. In one embodiment, the method uses a stochastic halftonescreen with an embedded watermark, i.e., a stochastic screen with aconjugate relationship to describe the location of the watermark. Theconjugate relationship determines the placement or location of the dotscorresponding to the digital watermark. A successive fill technique,such as the one described in U.S. Pat. No. 6,844,941 to G. Sharma et al.for Color Halftoning Using a Single Successive-Filling Halftone Screen,the contents of which are incorporated herein by reference in theirentirety, may be used to color the dots forming the digital watermarkwithin the halftone screen. For example, if an image has fourseparations, the successive fill technique will determine whether tocolor a particular dot black, cyan, magenta, yellow or not at all. Byusing a single halftone screen and a successive fill technique, thedifferent separations work together to produce the watermark with asignificantly higher signal to noise ratio. In addition, the use of asingle screen significantly improves the synchronization for thewatermark detection process.

For purposes of this disclosure, the term “screening” or “halftoning”refers to the process in which each pixel value of a 2D array of contonepixels is compared to one of a set of preselected thresholds (thethresholds may be stored as a 2D matrix and the repetitive patterngenerated by this matrix is considered a halftone cell), which producesa binary output at each pixel according to the result of the comparison.The matrix of threshold values is often referred to as a “screen”, andthe process of generating the binary image from the contone image usingthe screen is called “screening” or halftoning.

Also disclosed herein is a method for generating an authenticable colorimage, the color image including a plurality of color separations,wherein an authenticable image inserted in the color image is notreadily visually perceptible. According to one embodiment, the methodincludes providing a single halftone screen, wherein the single halftonescreen comprises a plurality of pixel locations with associatedthreshold values; wherein the halftone screen has a plurality of cells,each cell having a first region and a second region, each cell beingspatially offset from a neighboring cell by a first distance; wherein afirst region of a first cell is substantially identical to a firstregion of a second cell, and a second region of the first cell issubstantially conjugate to a second region of the second cell;halftoning image data corresponding to a first color separation usingthe single halftone screen, wherein a corresponding first set of screenpixel locations associated with a first set of threshold values arefilled by the first color separation; halftoning image datacorresponding to a second color separation using the single halftonescreen, wherein a corresponding second set of screen pixel locations arefilled by the second separation, the second set having threshold valuessuccessive to the first set of threshold values. When a first copy ofthe color image is spatially offset from a second copy of the colorimage by at least the first distance, at least a first cell of each ofthe first and second copy of the color image align with at least asecond cell of the first and second copy of the color image, andcontrast of the identical and conjugate regions become visible to formthe authentication image. The single halftone screen may be a stochasticscreen.

The method can be expanded to cover placement and coloring of the thirdand fourth color separations. The method may further include halftoningimage data corresponding to a third color separation using the singlehalftone screen, wherein a corresponding third set of screen pixellocations are filled by the third separation, the third set havingthreshold values successive to the second set of threshold values. For afourth color separation, the method may further include halftoning imagedata corresponding to a fourth color separation using the singlehalftone screen, wherein a corresponding fourth set of screen pixellocations are filled by the fourth separation, the fourth set havingthreshold values successive to the third set of threshold values. Thecolor separations may be black, magenta, cyan and yellow.

Further disclosed is a method for generating an authenticable colorimage, the color image including a plurality of color separations,wherein an authenticable image inserted in the color image is notreadily visually perceptible. The embodiment includes providing a singlestochastic halftone screen, the single stochastic halftone screencomprises a plurality of pixel locations with associated thresholdvalues; wherein the stochastic halftone screen has a plurality of cells,each cell having a first region and a second region, each cell beingspatially offset from a neighboring cell by a first distance; wherein afirst region of a first cell is substantially identical to a firstregion of a second cell, and a second region of the first cell issubstantially conjugate to a second region of the second cell;halftoning image data corresponding to the plurality of colorseparations using the stochastic halftone screen; further comprising,for each pixel, summing image values corresponding to the plurality ofcolor separations in a predetermined order; comparing sums image valuesof at least the first and second color separations to the thresholdvalues in the stochastic halftone screen; determining placement andcolor of the pixel in accordance with a predetermined relationship basedon the comparison; wherein, when a first copy of the color image isspatially offset from a second copy of the color image by at least thefirst distance, at least a first cell of each of the first and secondcopy of the color image align with at least a second cell of the firstand second copy of the color image, and contrast of the identical andconjugate regions become visible to form the authentication image.

For two separations, the predetermined relationship may be: if(i1>screen_threshold), printing a pixel with the colorant of the firstseparation; if ((i1+i2)>screen_threshold) and (i1<screen_threshold)),printing a pixel with the colorant of the second separation; and if((i1+i2−M)>screen_threshold), printing a pixel with the colorant of thesecond separation; where i1, i2 are the image values of the image datafor the first color separation and the second color separation,respectively, screen_threshold is the value of a threshold in thestochastic halftone screen, and M is the maximum threshold value. Notethat with the use of this method a given pixel may be printed with none,one, or both of the colorants. The predetermined relationship may beextended to a third separation and include: if((i1+i2+i3)>screen_threshold) and ((i1+i2)<screen_threshold)), printinga pixel with the colorant of the third separation; if((i1+i2+i3−M)>screen_threshold) and ((i1+i2−M)<screen_threshold),printing a pixel with the colorant of the third separation; and if((i1+i2+i3−M)>screen_threshold), printing a pixel with the colorant ofthe third separation.

A method for generating an authenticable color image, the color imageincluding a plurality of color separations, wherein an authenticationimage inserted in the color image is not readily visually perceptible,according to another embodiment of the method for generating anauthenticable color image, includes halftoning image data correspondingto a first color separation using a single halftone screen, wherein thesingle halftone screen includes means for generating an authenticationimage in a color image; and halftoning image data corresponding to asecond color separation using the single halftone screen and dotplacement information for the image data corresponding to the firstcolor separation to form a multicolor image; wherein halftoning of imagedata corresponding to the second color separation includes placing dotsfor the second color separation in thresholds of the halftone screenrelative to those thresholds occupied by the first color separation inthe halftone screen in accordance with a predetermined relationship.

Examples of predetermined relationships include: placing thresholds forthe second color separation adjacent to the thresholds of the firstcolor separation, placing thresholds for the second color separation ata predetermined distance from the thresholds of the first colorseparation, and generating a modified value for the second separation byadding the “halftone error” from the first separation and obtaining thesecond separation by screening the modified value for this separation.For example, halftoning of image data corresponding to the second colorseparation may include placing halftone dots for the second colorseparation in thresholds of the stochastic halftone screen determinedby: determining a halftone error between the first color separationimage data and the second color separation image data; and adding thehalftone error to the second color separation image data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a “conjugate” pair of binary patterns generated usinga monochrome conjugate halftone screen;

FIG. 2 illustrates the output of overlaying the two halftone imagesshown in FIG. 1;

FIG. 3 illustrates the output of overlaying two identical halftoneimages, shown as the left hand pattern of FIG. 1;

FIG. 4 illustrates the output of overlaying two halftone imagesgenerated by the pair of halftone screens described in Table 2;

FIG. 5 illustrates the output of overlaying two halftone imagesgenerated by the pair of halftone screens described in Table 3;

FIG. 6 illustrates the result of combining the output of FIG. 4 in cyanand the output of FIG. 5 in magenta;

FIG. 7 illustrates the result of combining in yellow and blue (left) andred and green (right);

FIG. 8 illustrates halftone images generated by an input with CMYKvalues K=C=M=Y=32 and the screens shown in Table 2, usingsuccessive-filling in colorant order K, M, Y, C;

FIG. 9 illustrates the output of overlaying the two halftone images inFIG. 8;

FIG. 10 illustrates a watermark to be included into an image; and

FIG. 11 illustrates a bias tile incorporating the watermark of FIG. 10.

DETAILED DESCRIPTION

One embodiment of a method for generating an authenticable color imageallows for a color pattern to be used on a color document, where thecolor pattern can be generated using a stochastic halftoning process toproduce a desirable image. Using such techniques, the random nature ofthe stochastic screen affords the opportunity to include a uniqueauthentication procedure in conjunction with correlations betweendifferent stochastic screens. As a primer to the principles of colorstochastic halftone screening, monochrome stochastic halftone screeningis discussed below. In various exemplary embodiments, this method forgenerating an authenticable color image uses the stochastic screeningmethod described in U.S. Pat. No. 5,673,121 to Wang, hereby incorporatedby reference in its entirety.

Each location in an image may be called a “pixel” or “dot”. In an arraydefining an image in which each item of data or image signal provides avalue, each value indicating the color of a location may be called a“pixel value” or “dot value”. In monochrome stochastic halftonescreening of monochromatic documents, halftone images are generated fromconstant gray-scale inputs by a screen matrix with N elements. If theoverlap between adjacent pixels is ignored, a screen cell with n blackpixels and N−n white pixels simulates the input with a gray scale (g)equal tog=(N−n)/N,where 0≦n≦N, or 0≦g<1. The visual appearance of this pattern depends onwhether the black pixels or the white pixels are minorities. If theblack pixels are minorities, for example, 0.5≦g≦1.0, the best visualappearance of the halftone pattern occurs when all black pixels are“evenly” distributed, in other words, each black pixel should “occupy”1/n, or 1/(1−g)N, fraction of the total area of the screen. Therefore,the average distance of adjacent black pixels should be equal toα(1−g)^(−1/2), where α is independent of gray levels. On the other hand,if the white pixels are minorities, i.e., 0≦g≦0.5, each white pixelshould “occupy” 1/(N−n) or 1/gN, fraction of the total area and theaverage distance of adjacent white pixels should be equal to αg^(−1/2).An idealized stochastic dithering screen is defined as a threshold maskgenerating halftone images, which satisfy the above criterion for allgray levels.

In general, input gray-scale images are specified by integer numbers,G(x, y), where 0≦G≦M. As a result, the dithering screen should have Mdifferent threshold values spanning from zero to M−1. Moreover, at eachlevel, there should be (N/M) elements having the same threshold value T.The ultimate goal of designing a stochastic screen is to distribute thethreshold values T so that the resulting halftone images are as close aspossible to the ones generated by an idealized stochastic screen.

Choosing an arbitrary pair of pixels from the dithering screen, it isassumed that the threshold values for these two pixels should beT₁=T(x₁, y₁) and T₂=T(x₂, y₂), respectively, where (x₁, y₁) and (x₂,y₂), are the coordinates of these pixels. As the result of dithering aconstant input G, the outputs B₁=B(x₁, y₁) and B₂=B(x₂, y₂) have thefollowing possible combinations:

B₁ = 1  and  B₂ = 1,  if  G ≥ T₁  and  G ≥ T₂;B₀ = 1  and  B₂ = 0,  if  G < T₁  and  G < T₂; B₁ ≠ B₂,where B=1 represents a white spot and B=0 represents a black spot forprinting. When one output pixel is black and another is white, thedistance between these two pixels is irrelevant to the visual appearancefor the reasons outlined above. When both pixels are white, the visualappearance under the following case must be considered:

If M/2≧G, G≧T₁, and G≧T₂.

In this case, both output pixels are white, and white spots areminorities. Therefore, the corresponding distance between (x₁, y₁) and(x₂, y₂) is relevant to the visual appearance of the halftone images.According to the analysis outlined above, this distance is greater orequal to αg^(−1/2), or α(G/M)^(−1/2), for outputs of an idealizedstochastic screen. Among all G under this case, the critical case of Gis the smallest one, or G_(c)=Max(T₁, T₂), which requires the largestdistance between the two pixels (x₁, y₁) and (x₂, y₂).

Similarly, when both dots or pixels appear as black dots or pixels, thevisual appearance under the following case must be considered:

If G≧M/2, G<T₁ and G<T₂.

Among all G under this case, the largest G is given by G_(c)=Min(T₁,T₂),which requires the largest distance α(1−G_(c)/M)^(−1/2) between (x₁, y₁)and (x₂, y₂).

Mathematically, a merit function q(T₂, T₂) can be used to evaluate thedifference between the idealized stochastic screen and the chosen one.For example, the following choice (Eq. 1) may be used:q(T ₁ ,T ₂)=exp(Cd ² /d _(c) ²),  (1)where

$\begin{matrix}{{d^{2} = {\left( {x_{1} - x_{2}} \right)^{2} + \left( {y_{1} - y_{2}} \right)^{2}}};} & \; \\{{d_{c}^{2} = {M/\left\lbrack {M - {{Min}\left( {T_{1},T_{2}} \right)}} \right\rbrack}},} & {{{{if}\mspace{14mu} T_{2}} > {{M/2}\mspace{14mu}{and}\mspace{14mu} T_{1}} > {M/2}},} \\{{d_{c}^{2} = {{M/{Max}}\left( {T_{1},T_{2}} \right)}},} & {{{{if}\mspace{14mu} T_{2}} \leq {{M/2}\mspace{14mu}{and}\mspace{14mu} T_{1}} \leq {M/2}},} \\{{d_{c}^{2} = 0},{i.e.},{q = 0},} & {{elsewhere};{{and}\mspace{14mu} C\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{{constant}.}}}\end{matrix}$

Since a dithering screen is used repeatedly for halftoning images largerthan the screen, for any chosen pair of pixels from the ditheringscreen, the closest spatial distance in corresponding halftone imagesdepends on the dithering method and should be used for the meritfunction. The overall merit function should include contributions of allpossible combinations. In an experiment, the summation of q(T₁, T₂) wasfor optimization, i.e.:Q=Σq(T ₁ ,T ₂),  (2)where Σ for all (x_(i), y_(1)≠(x) ₂, y₂).

The design of stochastic screens then becomes a typical optimizationproblem. When the threshold values of a chosen screen are rearranged,the merit function can be evaluated to determine the directions andsteps. Many existing optimization techniques can be applied to thisapproach. The simplest method is to randomly choose a pair of pixels andswap threshold values to see if the overall merit function Q is reduced.Since only those Q values related to the swapped pair need to berecalculation, the evaluation of Q does not consume significantcomputation time. All initial threshold values were randomly chosen by astandard random number generator.

Alternatively, the threshold assignments from an existing screen may beused. Besides the Gaussian function described by Eq. (1) as the meritfunction, other functions were tested, such as the Butterworth functionand its Fourier transform. Other optimization functions are possible.For each iteration, a pair of pixels was randomly chosen from thedithering screen, their threshold values swapped and the change of themerit function Q was calculated. If Q is not reduced, the thresholdvalues are restored. Otherwise, the next iteration is performed. Theoptimization process continues until a satisfied distribution ofthreshold values is achieved.

The issues discussed above regarding monochrome stochastic screens canbe developed to produce an invisible color watermark in a halftonedcolor document in accordance with various exemplary embodiments of thestochastic halftone screening methods according to this method forgenerating an authenticable color image.

U.S. Pat. No. 5,790,703 describes a method for generating watermarks inblack and white halftone printing using conjugate stochastic screens,and is incorporated herein by reference in its entirety. Two screens,T₁(x, y) and T₂(x, y), having the same size and the same shape, areconjugate, if for all elements (x, y) the corresponding pair ofthreshold values have the following relation (Eq. 3):T ₁(x,y)+T ₂(x,y)=M,  (3)where M is the number of total possible levels. By the thresholdingrule, which defines the binary status of the output B(x, y) based on therelation between an input value G(x, y) and the threshold value T(x, y)provides:

B(x, y) = 1,  if  G(x, y) ≥ T(x, y);B(x, y) = 0,  if  G(x, y) < T(x, y).It is interesting to notice that if the input image has a constantvalue, G(x, y)=M/2, the two binary outputs B₁(x, y) and B₂(x, y),generated by two conjugated screens T₁(x, y) and T₂(x, y) in Eq. 3, areexactly binary complement for all pixels. In other words, any blackpixel of B₁ has a corresponding white pixel of B₂ at the same pixellocation (x, y), and vice versa. If the input level G(x, y)<M/2, thebinary complement relation between B₁ and B₂ is still true for all whitespots, as minorities in this case. If G(x, y)>M/2, the binary complementrelation between B₁ and B₂ is true for all black spots, also asminorities in this case.

From the previous discussion on stochastic screens, it is not difficultto see that the conjugate screen T₂(x, y), of a well-designed stochasticscreen T₁(x, y), is also a well-designed stochastic screen, because forevery output level of T₂ there is a corresponding level of T₁, which isoptimized during the screen design process. The principal difference isthat if the level of T₁ is with black minorities, the correspondinglevel of T₂ is with white minorities, and similarly so for T₁ with whiteminorities. Consider the following two cases:

In a first example, two identical halftone images are generated using astochastic screen T₁(x, y) and printed on two transparencies,respectively. If the two transparencies are laid over each other andviewed in a show-through mode, the overall appearance depends on therelative position between the two halftone images. The maximal, or thebrightest, show-through can be obtained only with a perfectpixel-to-pixel alignment of the two images without any lateral shift orrotation. It should be appreciated that this statement is an analogue ofa two-dimensional auto-correlation of the halftone image. The maximalshow-through corresponds to the peak value of the auto-correlation, orin other words, the positive peak of the correlation.

In another example, two halftone images are generated by two conjugatedstochastic screens, T₁(x, y) and T₂(x, y) defined by Eq. 3,respectively. The cross-correlation between the two halftone images,generated by two conjugated screens, behaves opposite to theauto-correlation described above such that, after the two halftoneimages are laid over each other and perfectly aligned, the overallappearance reaches the minimal, or the darkest, show-through.Mathematically, this corresponds to a negative peak of thecross-correlation, or simply, the negative peak of the correlation.

These two examples can be relatively combined so that some portions ofthe second halftone image are generated by using the conjugate screenT₂(x, y) while the remaining portion of the second image are generatedby the same stochastic screen T₁(x, y), as used to generate the firsthalftone image. Laying a transparency of the second image over the firstone, a strong contrast occurs between the brightest and the darkestshow-through.

Practically, combining the two portions of the second halftone imagedescribed above can be realized by designing a new stochastic screenT₂(x, y), which has the same shape and size as the first stochasticscreen T₁(x, y). A portion of the new stochastic screen T₂ is madeconjugate to the corresponding portion of the first stochastic screen T₁while other portion of the new stochastic screen T₂ is made identical toa portion of the first stochastic screen T₁. By modifying theoptimization condition for stochastic-screen design as described, forexample, in U.S. Pat. No. 5,673,121, it is possible to make the boundarybetween the two portions of by the second screen visually seamless.Therefore, the halftone images generated by the new stochastic screen T₂appear just as good as halftone images generated by the first stochasticscreen T₁. Although the watermark, defined by the shape of the portionfor the conjugate relation, is visually imperceptible, the informationis hidden, or incorporated into the halftone images generated by thestochastic screen in a manner according to the degree of correlation.

A concrete example of the monochrome technique described in U.S. Pat.No. 5,790,703 will be described: Define two conjugate halftone screensas two thresholding masks having identical shape and size and satisfyingsuch conjugate relation that T1(i, j)=255−T2(i, j) for all correspondingpixels (i, j), where T1 and T2 are the thresholding values of the twomasks, respectively. An exemplary pair of conjugate halftone screens isshown in Table 1 (note that the halftone screens in Table 1 can beconsidered two halftone cells of a single halftone screen). Note thatthe sum of two values in any pair of corresponding pixels shown in Table1 is 255.

TABLE 1 A pair of conjugate halftone screen cells.

If an input with a constant level 128 is halftoned by the conjugatescreen cells shown in Table 1, the result will be a “conjugate” pair ofbinary patterns 110A, 110B as shown in FIG. 1. By overlaying the twobinary patterns in FIG. 1, it is possible to obtain a complete blackpattern as shown in FIG. 2. On the other hand, if two identical halftonepatterns are overlaid together, the output is exactly the same binarypattern as the overlaid patterns. For example, FIG. 3 shows the resultof overlaying two identical patterns as the left binary image in FIG. 1.

Consider the pair of halftone screen cells, as shown in Table 2, withthe upper three rows (a first region of a first halftone cell and afirst region of the second halftone cell) of the two screens/cells areconjugate while the lower three rows (a second region of the firsthalftone cell and a second region of the second halftone cell) areidentical.

TABLE 2 A pair of halftone screens, with a conjugate upper half and anidentical lower half.

The overlaying of two binary patterns generated by these two screens anda constant input 128 will appear as the pattern shown in FIG. 4.

Similarly, a pair of halftone screen cells with an identical first threerows (second region) and a conjugate second three rows (first region),shown in Table 3, will generate an overlaying pattern as shown in FIG.5.

TABLE 3 A pair of halftone screens, with an identical upper half and aconjugate lower half.

The method described in U.S. Pat. No. 6,731,409 extends the monochromeconjugate screen method described in U.S. Pat. No. 5,790,703 forgenerating monochrome watermarks for color halftoning to create colorcontrast by using combinations of conjugate screens and identicalscreens (a different halftone screen is used for each color separation).For example, apply the conjugate halftone screen shown in Table 2 to onechannel, say cyan, and apply the conjugate halftone screen shown inTable 3 to another channel, say magenta. The result provides the highestcontrast between cyan and magenta. FIG. 6 illustrates the result ofcombining the output of FIG. 4 in cyan and the output of FIG. 5 inmagenta, where it is assumed no yellow and black inputs are applied.Since most applications of color halftoning have 3 or 4 color channels,other variations of combining conjugate screens and identical screensare possible and two examples of the results are shown in FIG. 7. FIG. 7illustrates the result of combining in yellow and blue (left) and redand green (right). As noted earlier, this method sometimes producesimages having less than desirable contrast and low signal-to-noiseratios for detection.

The method for generating an authenticable color image described hereinextends the single conjugate halftone screen method to produce colordigital watermarks. The method for generating an authenticable colorimage proposes a significantly improved system for color digitalwatermarks using a single halftone screen for all color separations. Inone embodiment, the method uses successive filling with a stochasticscreen designed with an embedded watermark, the different separationswork together in producing the watermark. The resulting halftone screenprocess produces a significantly higher signal to noise ratio for thewatermark. The method is applicable to single halftone screen techniquessuch as successive filling halftoning using stochastic screens andsimilar halftoning techniques. The method offers a significantimprovement in watermark signal to noise ratio over the previouslydisclosed color watermarking method.

When the method is applied to a stochastic halftone screen, theauthenticable color image (or watermark) is embedded in the singlestochastic screen. The stochastic halftone screen includes a pluralityof cells, each cell having at least one first region and at least onesecond region, wherein each cell is spatially offset from a neighboringcell by at least a first distance; wherein a first region of a firstcell of the stochastic halftone screen is substantially identical to afirst region of a second cell of the stochastic halftone screen, and asecond region of the first cell of the first stochastic halftone screenis substantially conjugate to a second region of the second cell of thefirst stochastic halftone screen. The conjugate region provides thewatermark in the resulting halftone image. When the watermark is to bedetected from a scan or other electronically captured image of theprinted color document, distortions in the printing and scanning processcan make the alignment difficult. In such circumstances, the identicalregions of the two cells can help determine the alignment between thescans of the regions corresponding to the two cells and thereby aid theprocess of “synchronization” of the image with a shifted version for thepurpose of watermark detection. In this respect, the disclosed systemand method are also advantaged because the same screen is utilized forthe different separations and therefore more of the printed dotlocations in the two cells will be common in the identical screenregions.

The same stochastic halftone screen is used for all color separations.Successive-filling is a technique proposed for color halftoning whereina single halftone screen is used for multiple separations, theseparations are allocated “successive levels” of the screen. Thus if theinput CMYK (Cyan, Magenta, Yellow, Black) color image is spatiallyconstant with values for the separations arranged in a specific order asi1, i2, i3, and i4 (for instance, typically in order darkest to lightestthese would correspond to K, M, C, and Y, respectively) the first i1levels of the halftone screen i.e. 1 through i1 are used for the firstseparation, the next i2 levels of the halftone screen, i.e. i1+1 throughi1+i2 are used for the second separation, the next i3 levels of thehalftone screen, i.e. i1+i2+1 through i1+i2+i3 are used for the thirdseparation, and the next i4 levels, i.e. i1+i2+i3+1 through i1+i2+i3+i4are used for the fourth separation. It is understood that in thisprocess if the levels of the halftone screen are exhausted, they arere-used employing exactly the same order as for the initial screen.

The successive fill process may be mathematically performed usingseveral equivalent methods. One method is the method described in U.S.Pat. No. 6,844,941. An alternate method may include the following steps:summing image values corresponding to the plurality of color separationsin a pre-determined order; comparing the image value sums of at leasttwo separations to the stochastic screen thresholds; and for each dot,selecting the dot's color and placement based on results of thecomparisons. The following relationship may also be used:

-   -   if (i1>screen_threshold), printing a dot having the color of the        first separation;    -   if (((i1+i2)>screen_threshold)) and (i1<screen_threshold))),        printing a dot having the color of the second separation; and    -   if ((i1+i2−M)>screen_threshold), printing a dot having the color        of the second separation;

where i1, i2 are the image values of the image data for the first colorseparation and the second color separation, respectively,screen_threshold is the value of a threshold in the stochastic halftonescreen, and M is the maximum threshold value.

This relationship may be further extended to three and four colorseparations. For the third separation, the relation is given by:

-   -   if (((i1+i2+i3)>screen_threshold)) and        ((i1+i2)<screen_threshold)), printing a dot having the color of        the third separation; and if (((i1+i2+i3−M)>screen_threshold))        and ((i1+i2−M)<screen_threshold)), printing a dot having the        color of the third separation;        -   if ((i1+i2+i3−2*M)>screen_threshold), printing a dot having            the color of the third separation;

Similarly, the process for the fourth separation is

-   -   if (((i1+i2+i3+i4)>screen_threshold) and        ((i1+i2+i3)<screen_threshold)), printing a dot having the color        of the third separation; and    -   if (((i1+i2+i3+i4−M)>screen_threshold) and        ((i1+i2+i3−M)<screen_threshold)), printing a dot having the        color of the third separation;    -   if (((i1+i2+i3+i4−M)>screen_threshold) and        ((i1+i2+i3−M)<screen_threshold)), printing a dot having the        color of the third separation;    -   if ((i1+i2+i3−M)>screen_threshold), printing a dot having the        color of the third separation.

Since successive filling uses a single halftone screen for all the colorseparations, the watermarks in the different separations act in concert(unlike the color contrast halftone watermarks where the independentseparations act independently). As a result the watermark pattern has ahigher signal to noise ratio and the shift for obtaining the watermarkpattern is also estimated more easily from the scan of a print bearingthe embedded watermark.

For illustrating the method for generating an authenticable color image,consider the pair of screens shown in Table 2 and consider the result ofhalftoning a region with input CMYK values K=C=M=Y=32, using successivefilling with the order black, magenta, cyan, yellow (increasinglightness order as is common for successive filling). The result ofhalftoning this color region with these two halftone screens is shown inFIG. 8 where the left hand side corresponds to the result of halftoningwith the screen on the left in Table 2 and the right side corresponds tothe result of halftoning with the screen on the right in Table 2. Theresult of overlaying these two halftone images (in the process ofhalftone detection) is shown in FIG. 9, where the image on the left handside indicates the overlay in color and the image onto the right showsthe result of detecting the presence of a halftone dot (of any colorant)on each pixel—for instance through the process of taking the minimum ofRGB values in each pixel. From FIG. 9, it can be seen that the processof successive filling and detection of dots on each pixel makes thecolor halftoning watermark analogous to the black and white watermarkthereby significantly improving its detectability.

While the bitmaps presented here illustrate the improvements with themethod for generating an authenticable color image in halftone bitmaps,it is important to consider the full process of watermarking anddetection wherein the halftone bitmaps are printed and scanned prior todetection of embedded information. Experiments were conducted toevaluate the performance of the new scheme and to compare it with colorcontrast watermarking.

In order to evaluate the proposed method for generating an authenticablecolor image and to compare its performance with the color-contrastwatermarking method, an experiment was performed. A monochrome halftonestochastic screen with an embedded conjugate watermark in the shape ofan X was designed. Two watermarked halftone bitmaps were created usingthis screen: the first bitmap used the existing color-contrastwatermarking scheme disclosed in U.S. Pat. No. 6,731,409, herebyincorporated by reference in its entirety, and the second used themethod for generating an authenticable color image with successivefilling as described above.

Aspects of the disclosed system may be found in a color xerographicprinting system. For example, the halftone bitmaps were printed on aPhaser 850 printer from Xerox Corporation at 300 dpi resolution. Oneprint was printed with a color contrast watermark (U.S. Pat. No.6,731,409) and a second print was printed with the successive fill colorwatermark generated by the method for generating an authenticable colorimage. The two printed images were scanned using a UMAX Powerlookdesktop scanner 300 dpi resolution and a watermark detection algorithm,carried out on a workstation with hardware, software and circuitry(memory, processor, etc.) suitable for performing digital imageprocessing operations (e.g., halftoning), was executed on the scans. Theresult of the watermark detection algorithm on the color contrastwatermarked image showed that the watermark was extremely faint andvisible only in certain regions. The result of the watermark detectionalgorithm on the successive fill color watermark showed the watermark“X” pattern was clearly visible over most smooth regions of the image.From the results, it is clear that the proposed successive filling colorwatermark offers a very significant improvement in detectability incomparison to existing methods.

The signal to noise ratio (SNR) of each watermark was also estimatedusing the watermark detection algorithm. The average SNR for the colorcontrast watermarking scheme is 0.88 and the average SNR for thesuccessive filling watermarking scheme is 3.34. The much higher SNR forthe successive filling watermarking is consistent with the visualresults and indicates the significant improvement in performance offeredby the method for generating an authenticable color image. Theexperimental results indicate that the method for generating anauthenticable color image provides a very significant improvement incolor halftone watermarks bringing this technology much closer topractical applications, most of which involve color.

While the description thus far has been directed to watermarking ofcolor halftone images generated using stochastic halftone screens, themethod of the proposed invention may also be applied for color dataembedding using error diffusion. In one embodiment, this can be realizedby adapting the monochrome halftone data embedding method disclosed inU.S. Pat. No. 6,636,616 to color using a successive filling techniquesuch as the one described in U.S. Pat. No. 6,721,063. Both U.S. Pat. No.6,636,616 and 6,721,063 are hereby incorporated by reference in theirentirety.

This process is best illustrated by means of an example. It is to benoted that the example is for illustrative purposes and in actualpractice several different realizations are possible. Suppose thepattern illustrated in FIG. 10 is to be embedded in a color halftoneimage consisting of CMYK planes. Embedding is accomplished by halftoningthe multiple separations using a joint error diffusion method andintroducing a bias in the error diffusion image halftoningprocess—through the addition of a watermark pattern dependent bias. Anexemplary bias pattern is illustrated in FIG. 11, where it is twice thesize of the watermark pattern to be introduced. The pattern is zeroexcept in regions corresponding to the watermark pattern where it istakes complementary values in the left and right halves 1110 and 1112,respectively. This bias is added to the threshold for error diffusion.The values t and u may be chosen for instance as t=−u=64. The additionof this bias pattern to the threshold in the halftoning process favorsthe placement of printed halftone dots on pixels labeled as u's (becausethe threshold is lowered in these regions) and discourages the placementof dots in the pixels where the bias is t (because the threshold israised in these regions). Since the bias pattern in FIG. 11 has the t'sand the u's transposed on the right half in relation to their locationsin the left half, the printed halftone dots in the output would have apropensity to lie in complementary locations. If the halftone image (ora suitable scan of the printed halftoned image) is shifted horizontallyto the right by a displacement corresponding to half the size of therectangle indicated in FIG. 11 and overlaid on itself, the dots in theregions corresponding to the “+” shaped watermark pattern would tend tolie on different locations while the dots in the regions outside the “+”shaped watermark pattern would tend to be randomly located in theshifted version. Thus the “+” shaped watermark pattern would appear as adarker region. The contrast of the watermark may be further improved bybiasing the blank pixels shown in FIG. 2 to make dots in those regionshave a higher propensity to lie in identical locations. The descriptionthus far does not indicate how the color planes are accommodated;accordingly one algorithmic embodiment of the manner in which this maybe accomplished is now presented:

Consider a CMYK (Cyan, Magenta, Yellow, Black) color image where thevalues for the separations are indicated in a specific order at pixellocation (x,y) as i1(x,y), i2(x,y), i3(x,y), and i4(x,y), which areassumed to be distributed between 0 and 1 for our description in thispart. Then the process may be described as follows:

Compute the sum of all colorants s(x,y)=i1(x,y)+i2(x,y)+i3(x,y)+i4(x,y)

Apply a multilevel error diffusion to the sum to quantize each pixellocation to 0 (no dots), 1 (one colorant dot), 2 (two colorant dots), 3(three colorant dots) and 4 (four colorants dots). This process can beachieved for instance by computing for each pixel a modified valuei(x,y)=s(x,y)+e(x,y) where e(x,y) is the error diffused to the location(x,y) from previously processed locations in accordance with well-knownerror diffusion methods. Quantize i(x,y) to the four levels to obtain anoutput value o(x,y) as follows:

${o\left( {x,y} \right)} = \left\{ \begin{matrix}0 & {{{if}\mspace{14mu}{i\left( {x,y} \right)}} \leq {0.5 + {w\left( {x,y} \right)}}} \\1 & {{{{if}\mspace{14mu}{w\left( {x,y} \right)}} + 0.5} < {i\left( {x,y} \right)} \leq {{w\left( {x,y} \right)} + 1.5}} \\2 & {{{{if}\mspace{14mu}{w\left( {x,y} \right)}} + 1.5} < {i\left( {x,y} \right)} \leq {{w\left( {x,y} \right)} + 2.5}} \\3 & {{{{if}\mspace{14mu}{w\left( {x,y} \right)}} + 2.5} < {i\left( {x,y} \right)} \leq {{w\left( {x,y} \right)} + 3.5}} \\4 & {{{if}\mspace{14mu}{i\left( {x,y} \right)}} > {{w\left( {x,y} \right)} + 3.5}}\end{matrix} \right.$

where w(x,y) is the bias at pixel location (x,y) determined inaccordance with the watermark pattern as indicated above. Diffuse thequantization error i(x,y)−o(x,y) to the neighbors not processed yet inaccordance with established error diffusion procedures.

Apply independent error diffusion with constraints to the individualseparations to determine the colorants to be included as each of thelocations identified in step 2. For example for the first separation,compute i1′(x,y)=i1(x,y)+e1(x,y) where e1(x,y) is the error diffused tothe location (x,y) from previously processed locations of the firstseparation in accordance with well-known error diffusion methods.Similarly calculate i2′(x,y), i3′(x,y), i4′(x,y). For quantization ofthe values however use the constraints on the number of pixels to beprinted that were previously established, i.e., if o(x,y) is non-zero,pick the largest o(x,y) values from i1′(x,y), i2′(x,y), i3′(x,y),i4′(x,y) and set the corresponding values for the corresponding outputso1′(x,y), o2′(x,y), o3′(x,y), o4′(x,y) as 1 leaving other values as 0.For each of the separations compute and diffuse the quantization errorto the neighbors not processed yet in accordance with established errordiffusion procedures.

It will be appreciated that various of the above-disclosed embodimentsand other features and functions, or alternatives thereof, may bedesirably combined into many other different systems or applications.Also, various presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A method for generating a color image carrying an embedded watermarkimage, the color image including a plurality of separations, wherein awatermark image inserted into the color image is not readily visuallyperceptible, comprising: a. providing a single halftoning process forthe plurality of separations, wherein the halftoning process includes aperiodic tile, each tile offset from its neighbors by a fixeddisplacement, and where said periodic tile includes a first region and asecond region and a first part of the first region is substantiallyidentical to a corresponding part of the second region, and a secondpart of the first region is substantially conjugate to a correspondingpart of the second region; b. halftoning image data from the color imageusing the single halftoning process wherein a set of pixel locationsassociated with a first threshold value are filled by a first of theplurality of separations; c. halftoning the image data using the singlehalftoning process wherein a second set of pixel locations associatedwith second of the plurality of separations, the second set of pixellocations are filled successively to the first set of pixel locations,such that when a first copy of the color image is spatially offset froma second copy of the color image by an integral multiple of the fixeddisplacement, at least the first part of the first region aligns with asecond part of the second region and a second part of the first regionaligns with a first part of the second region and contrast of theidentical and conjugate first and second parts become visible to revealthe watermark image.
 2. The method of claim 1, wherein the singlehalftoning process comprises comparing the image values to a stochasticscreen of thresholds.
 3. The method of claim 2, further comprisinghalftoning image data corresponding to a third of the plurality ofseparations using the single stochastic screen of thresholds, wherein athird set of pixel locations associated with third of the plurality ofseparations, the third set of pixel locations are filled successively tothe second set of pixel locations.
 4. The method of claim 3, wherein thefirst color separation represents black, the second color separationrepresents magenta and the third color separation represents cyan. 5.The method of claim 2, further comprising: determining a firstseparation halftone error as the difference between the first colorseparation image data value and first color separation halftone imagedata output value; and adding the first separation halftone error to thesecond color separation image data prior to halftoning image datacorresponding to the second color separation.
 6. The method of claim 1wherein the single halftoning method further comprises an errordiffusion screen.
 7. The method of claim 6 wherein the periodic tilecomprises adding a tile of bias values to the theshold of the errordiffusion screen, wherein at least one region of the tile raises thethreshold, and at least one remaining region of the tile lowers thethreshold.
 8. The method of claim 6 wherein the error diffusion screencomprises a. first computing an error diffused value based upon a sum ofthe values of all of the plurality of separations to determine how manyof the separations are to be printed at each pixel location, b.independently performing an error diffusion on each separation, and c.printing only the number of separations that has been determined in a.9. The method of claim 8 wherein the separations to be printed areprinted in order of increasing luminance impact, with the darkestseparations printed first.
 10. A method for generating an authenticablecolor image, the color image containing a plurality of separations,wherein an authenticable image inserted into the color image is notreadily visible, comprising: receiving color image data containing aplurality of separations for the image to be authenticated; halftoningthe color image data to generate a binary color image therefrom, whereinthe halftoning includes a stochastic screen; adding a bias pattern tothe halftoning, wherein the bias pattern is periodically and regularlyrepeated throughout the halftoned image, wherein the bias patterncomprises a stochastic mask with a first region and a second region,where a first part of the first region is substantially identical to acorresponding part of the second region, and where a second part of thefirst region is substantially conjugate to a corresponding part of thesecond region; and controlling the successive halftoning of each of theplurality of separations, wherein each successive separation fillspixels successive to the previously halftoned separation.
 11. The methodof claim 10 wherein halftoning further comprises determining a firstseparation halftone error between the first color separation image dataand first color separation output halftone image data and adding thefirst separation halftone error to the second color separation imagedata prior to halftoning image data corresponding to the second colorseparation.
 12. The method of claim 10 wherein halftoning comprisesperforming an error diffusion halftoning on the color image data. 13.The method of claim 12 wherein adding a bias pattern adds a bias valueto the threshold of the error diffusion screen, which in some regionsraises the threshold, and in the remaining regions lowers the threshold.14. The method of claim 12 wherein halftoning includes performing athree step error diffusion halftoning on the color image.
 15. The methodof claim 14, where the halftoning first computes a multi level errordiffused value based the sum of the values of all of the plurality ofseparations to determine a number of the separations to be printed ateach pixel location, and then computes an error diffused value for eachseparation, and finally outputs only the number of separations that hasbeen determined in the first step.